Constraining magnetization of gamma-ray bursts outflows using prompt emission fluence

Abstract

Considered here is the acceleration and heating of relativistic outflow by local magnetic energy dissipation process in Poynting-flux dominated outflow. Adopting the standard assumption that the reconnection rate scales with the Alfven speed, I show here that the fraction of energy dissipated as thermal photons cannot exceed (13 (gamma) over cap -14)(-1) = 30% (for adiabatic index (gamma) over cap = 4/3) of the kinetic energy at the photosphere. Even in the most radiatively efficient scenario, the energy released as non-thermal photons during the prompt phase is at most equal to the kinetic energy of the outflow. These results imply that calorimetry of the kinetic energy that can be done during the afterglow phase could be used to constrain the magnetization of gamma-ray bursts (GRB) outflows. I discuss the recent observational status and its implications on constraining the magnetization in GRB outflows.